C ++用迭代器初始化向量

Question

I am attempting to read a file of unsigned characters into a vector. My slightly roundabout way of doing this is by creating an input file stream, reading that data into a stringstream, and initializing a vector using the contents of the stringstream. There is probably a much better way to do this, and my existing code causes a segmentation fault using certain files.

string file_name = "output/comp" + to_string(start_id) + ".xml";

if( !file_exists(file_name))
{
    cout << "File could not be found." << endl;
    return 0;
}

ifstream ifs(file_name);
stringstream ss;
ss << ifs.rdbuf();
ss >> noskipws;

vector<unsigned char> raw_data(ss.str().begin(), ss.str().end());

I have included all the necessary headers and using declarations, and file_exists() returns a boolean value indicating whether or not the file exists.

My question is this: Why is the above code incorrect, and what is the best method for correctly accomplishing the same objective?

Answer 1

std::stringstream::str() returns a std::string . That is, it's a temporary string.

So ss.str().begin() returns the beginning iterator for a temporary string, and ss.str().end() returns the ending iterator of a different temporary string, because both ss.str() calls are returning different temporary strings. You can't mix iterators between different objects!

To fix this, use one string:

std::string str = ss.str();

And now just pass str.begin() and str.end() .

Answer 2

The problem with

vector<unsigned char> raw_data(ss.str().begin(), ss.str().end());

is that str() returns a temporary string. This means the string you use for begin is not the same string you use for end . Since the iterators do not refer to the same string you cannot use them together.

Instead of using a strignstream we can read the file directly into the vector using istream_iterator s like

std::ifstream ifs(file_name);
std::istream_iterator<unsigned char> start(ifs), end;
std::vector<unsigned char> raw_data(start, end);

Answer 3

i'm not exactly sure why that would cause a segfault, perhaps something in string stream, but with c++11 it's very easy to read a file into a string once you have any istream derivative:

std::string readFileAsString(std::istream& stream)
{
     return std::string(std::istreambuf_iterator<char>(stream), std::istreambuf_iterator<char>());
}

edit sorry, I forgot you wanted the bytes at the end

return std::vector<char>(std::istreambuf_iterator<char>(stream), std::istreambuf_iterator<char>());